1. Field of the Invention
The present invention relates to methods of manufacturing isotactic-atactic poly(olefin) compositions and isotactic-atactic poly(olefin)s produced thereby.
2. Background Art
Thermoplastic poly(olefin) elastomers, such as polypropylene (PP), based on an isotactic-atactic stereoblock microstructure have been the focus of interest for nearly 50 years in the continuing quest for ‘new materials from old monomers’ through the Ziegler-Natta polymerization of olefins (Natta, G. J., Polym. Sci. 34:531-549 (1959); Collete, J. W., et al., Macromolecules: 22, 3851-3858 (1989); Mallin, D. T., et al., J. Am. Chem. Soc. 112:2030-2031 (1990); Coates, G. W. and Waymouth, R. M., Science 267:217-219 (1995); Miller, S. A., Macromolecules 37:3983-3995 (2004)). Natta first proposed nearly half a century ago a new class of thermoplastic elastomer based on an isotactic-atactic stereoblock poly(propylene) microstructure (G. Natta, J. Polym. Sci. 34:531 (1959)). Subsequent efforts to obtain such materials under controlled conditions resulted in the production of elastomers that were determined to have an isotactic-atactic stereoblock microstructure (See, for example: J. W. Collette, C. W. Tullock, R. N. MacDonald, W. H. Buck, A. C. L. Su, J. R. Harrell, R. Mülhaupt, B. C. Anderson, Macromolecules 22:3851 (1989); D. T. Mallin, M. D. Rausch, Y. G. Lin, S. Dong, J. C. W. Chien, J. Am. Chem. Soc. 112:2030 (1990); and G. W. Coates, R. M. Waymouth, Science 267:217 (1995)). However, owing to the probabilistic way in which the microstructure evolves in these polymerizations, all the materials reported to date are actually stereochemical heterogeneous in nature; that is, the collection of polymer chains in the sample represents a distribution of stereochemical microstructures (L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 100:1253 (2000)).
Although a few strategies have now been reported to provide isotactic-atactic stereoblock poly(propylene) under controlled conditions, all the materials obtained to date are, in fact, heterogeneous in nature; being statistical mixtures of polymer chains representing a distribution of stereochemical microstructures, and some of these can be physically fractionated into multiple components. Furthermore, apart from some variation in the microstructure distributions that might be possible through control of monomer concentration, these strategies share the common distinction of being ‘one catalyst, one material,’ and therefore, access to the full range of isotactic-atactic stereoblock poly(olefin), and in particular PP, architectures that can be conceivably envisioned with respect to the length and distribution of stereoblock type remains out of reach. Finally, and perhaps most significantly, the mechanisms by which these isotactic-atactic stereoblock poly(olefin) materials are produced, as well as the exact nature of their microstructures, still remain either unclear or open to question (Gauthier, W. J. and Collins, S., Macromolecules 28:3779-3786 (1995); Busico, V.; et al., J. Am. Chem. Soc. 125:5451-5460 (2003)).
Manipulation of poly(olefin) stereochemical microstructure through ligand modifications of well-defined homogeneous Ziegler-Natta catalysts has provided a wealth of new materials; ranging from stiff or flexible plastics to elastomers (L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 100:1253 (2000); G. Müller, B. Rieger, Prog. Polym. Sci. 27:815 (2002); C. De Rosa, F. Auriemma, A. Di Capua, L. Resconi, S. Guidotti, I. Camurati, I E. Nifant'ev, I. P. Lashevtsev, J. Am. Chem. Soc. 126:17040 (2004)). However, the currently practiced ‘one catalyst—one material’ strategy has significant disadvantages and practical limitations for fine-tuning physical properties through either minor adjustments about a given microstructure, or for accessing a completely different microstructure altogether. Thus, not only is it a labor-intensive synthetic undertaking to prepare a large variety of catalyst structural variants that may, or may not, yield a desired microstructure, but even after several decades of effort, the ‘rational design’ of new catalysts that can produce a specific poly(olefin) microstructure is still out of reach; and even more so for non-metallocene based systems (see, for example: G. Talarico, V. Busico, L. Cavallo, J. Am Chem. Soc. 125:7172 (2003)).
Dynamic unimolecular processes that are competitive with propagation, such as site-isomerization in structurally constrained C1-symmetric ansa-bridged metallocenes (D. T. Mallin, M. D. Rausch, Y. G. Lin, S. Dong, J. C. W. Chien, J. Am. Chem. Soc. 112:2030 (1990); J. C. W. Chien, G. H. Llinas, M. D. Rausch, G.-Y. Lin, H. H. Winter, J. Am. Chem. Soc. 113:8570 (1991); W. J. Gauthier, J. F. Corrigan, N. J. Taylor, S. Collins, Macromolecules 28:3771 (1995); W. J. Gauthier, S. Collins, Macromolecules 28:3779 (1995); A. M. Bravakis, L. E. Bailey, M. Pigeon, S. Collins, Macromolecules 31:1000, (1998); U. Dietrich, M. Hackmann, R. Bernhard, M. Kling a, M. Leskelä, J. Am. Chem. Soc. 121:4348 (1999); E. J. Thomas, C. W. J. Chien, M. D. Rausch, Macromolecules 33:1546 (2000); S. A. Miller, J. E. Bercaw, Organometallics 21:934 (2002)), conformational flexibility in unconstrained ‘oscillating’ metallocenes (G. W. Coates, R. M. Waymouth, Science 267:217 (1995); S. Lin, R. M. Waymouth, Acc. Chem. Res. 35:765, (2002)) and ‘chain-end epimerization’ (V. Volkis, E. Nelkenbaum, A. Lisovskii, G. Hasson, R. Semiat, M. Kapon, M. Botoshansky, Y. Eishen, M. S. Eisen, J. Am. Chem. Soc. 125:2179 (2003)) or ligand sphere rearrangements in non-metallocenes (E. Smolensky, M. Kapon, J. D. Woollins, M. S. Eisen, Organometallics 24:3255 (2005)), can give rise to poly(propylene) materials having properties that result from varying degrees and patterns of stereoerror incorporation, such as in the case of elastomeric poly(propylene). Due to the intrinsically low energy barriers associated with these unimolecular processes, however, to date, the only means available by which to exert some level of external control in order to access a wider range of microstructures for a given catalyst has been to capitalize on the bimolecular nature of propagation (olefin complexation) by varying propylene pressure, and hence, the rate of propagation, Rp, vs. that of the unimolecular process (U. Dietrich, M. Hackmann, R. Bernhard, M. Kling a, M. Leskelä, J. Am. Chem. Soc. 121:4348 (1999)).
There is a need, therefore, for new methods of producing poly(olefin) compositions having defined microstructure. And there is a need for new isotactic-atactic poly(olefin) compositions that exhibit elastomeric properties.